2003 4.0, Replaced Thermostat Housing, now, it's hard to start only when cold?

Mirage775 · Mar 15, 2017

I replaced my complete thermostat housing this past Monday. I should've replaced the 2 sensors, but I didn't and that may be my problem. Anyway, after I finished replacing the housing and reinstalled everything. The truck fired up just fine! No leaks and ran great. It was about 46 degrees Monday afternoon. I drove to Autozone to return some things that afternoon and it started and ran fine, zero starting problems...

Tuesday morning, about 6am, it was 30 degrees and I got in my truck to head to work and it turned over, but never started! I tried to start it for 5-10 seconds, nothing. I turned the key off, then just turned it back on and it fired right up and ran fine! I work within 5 miles of my house and when I got to work, I shut it off and thought I'd try it again, to see if it hesitated starting, but it didn't! It fired right up! The heat didn't even have time to get hot on the way to work. 12 hours later, I left work and it did the same thing, with not starting immediately...

This morning, Wednesday, it was 20 degrees and it did the same thing as Tuesday. 9 hours later, when I got off work, it did it again...

I've talked to 2 different trusted mechanics and they're both stumped. One immediately said it was a fuel problem and to change my fuel pump and filter. I just changed both less than a year ago.

The other mechanic said it was due to the super low temps we've had. Or something in one of the two sensors in the thermostat housing, but he bet more on the weather...

I'm thinking that it's both the Temp Switch and Coolant Temp Sensor? I cannot find anything online, with 2 days of searching....

I had the TB off, so I replaced that gasket as well. I used CRC Electronic cleaner on every connector I removed as well.

It was running fine before I changed Thermostat Housings... Now, it's doing this. How can it not be related to something I changed?

If there's no replies by Friday, I'm just gonna take it all back apart and replace those sensors.

If anyone knows anything else about my problem, I would greatly appreciate any help?

gw33gp · Mar 16, 2017

As you probably already know, it is recommended to replace the two sensors when you replace the thermostat housing. A bad sensor or connection is most likely your problem.
You should try pushing down and wiggling the connector on both sensors first to make sure the connectors are fitting well/in place and see if that helps. Besides cleaning connectors, you should also use dielectric grease on them when reconnecting.
Just to make sure it is not a problem with the valve in the fuel pump, there is an easy test for that. Just turn the ignition switch to the run position (not start) for a couple seconds and turn it off. Repeat this 2 or 3 times and then try to start it. This will pump up the fuel pressure if the anti-back flow valve is not holding pressure. New parts can fail too.

RonD · Mar 16, 2017

+1 ^^^^

A gasoline engine is still a gasoline engine no matter what they add to the outside

Gasoline doesn't ignite with a spark.
yes, the movies guys take liberties with that but the laws of physics are the laws of physics, lol.

Only gasoline vapor will ignite with a spark, and the colder the gasoline is the less vapor it will have.
This is why gasoline engines needed a Choke to get them started on colder days, the colder it is then more "Choke" you needed.
A Choke plate on a carburetor restricted air flow into the engine, which increased vacuum while cranking engine, increased vacuum caused more gasoline to be sucked out of the Jets.
So Choke = add more fuel

Say you need 30% gasoline vapor in an engine for a spark to ignite it.
On a cold day gasoline has 15% vapor
So you need to suck in 2 times the "normal" amount of gasoline to get the 30% minimum to get engine to start.
These are made up % to show why gasoline engines need to be choked, you can look up the real numbers if you want

Once engine starts, cylinders heat up pretty fast so gasoline gets more vapor as it enters heat cylinders.
Compression ratio is also a factor as compression = heat, so older engines can be harder to start in colder weather simply because compression is lower.

Fuel injection has NO JETS.........so a Choke Plate won't work
But a gasoline engine still needs that 30% vapor for the spark to start a cold engine.
That's what the ECT(engine coolant temperature) sensor is for.
ECT sensor is only connected to computer, in 2003 and earlier.
There is also a SENDER near the ECT sensor, it is only used by the dash board temp gauge.
Around 2005 they did combine these, well they actually got rid of the SENDER, and the Computer sent engine temp to the dashboard temp gauge.

So the ECT sensor tells the Computer how much Choke(extra fuel) to apply to the cold engine, gasoline's vapor % can be calculated based on temp, as said you can look that up, Ford did, lol, so the computer knows how much extra gasoline to squirt into the engine to get the correct amount of vapor in the cylinder for spark to start the engine at any temperature.

So, yes check connector on ECT Sensor, it is an important sensor for cold starts, must have accurate temperature.
ECT sensor is monitored so would usually cause CEL(check engine light) if there was a problem.
But Computer can only monitor that it is changing as engine heats up, it can't tell if it is accurate for 20degF or 30degF before startup

And yes do the key on/key off a few times before starting, as a loss of fuel pressure when vehicle sits is not uncommon and more likely than the ECT sensor issue.
But since you just worked on that area of the engine could be ECT

Mirage775 · Jan 19, 2019

gw33gp said:
As you probably already know, it is recommended to replace the two sensors when you replace the thermostat housing. A bad sensor or connection is most likely your problem.
You should try pushing down and wiggling the connector on both sensors first to make sure the connectors are fitting well/in place and see if that helps. Besides cleaning connectors, you should also use dielectric grease on them when reconnecting.
Just to make sure it is not a problem with the valve in the fuel pump, there is an easy test for that. Just turn the ignition switch to the run position (not start) for a couple seconds and turn it off. Repeat this 2 or 3 times and then try to start it. This will pump up the fuel pressure if the anti-back flow valve is not holding pressure. New parts can fail too.

RonD said:
+1 ^^^^

A gasoline engine is still a gasoline engine no matter what they add to the outside

Gasoline doesn't ignite with a spark.
yes, the movies guys take liberties with that but the laws of physics are the laws of physics, lol.

Only gasoline vapor will ignite with a spark, and the colder the gasoline is the less vapor it will have.
This is why gasoline engines needed a Choke to get them started on colder days, the colder it is then more "Choke" you needed.
A Choke plate on a carburetor restricted air flow into the engine, which increased vacuum while cranking engine, increased vacuum caused more gasoline to be sucked out of the Jets.
So Choke = add more fuel

Say you need 30% gasoline vapor in an engine for a spark to ignite it.
On a cold day gasoline has 15% vapor
So you need to suck in 2 times the "normal" amount of gasoline to get the 30% minimum to get engine to start.
These are made up % to show why gasoline engines need to be choked, you can look up the real numbers if you want

Once engine starts, cylinders heat up pretty fast so gasoline gets more vapor as it enters heat cylinders.
Compression ratio is also a factor as compression = heat, so older engines can be harder to start in colder weather simply because compression is lower.

Fuel injection has NO JETS.........so a Choke Plate won't work
But a gasoline engine still needs that 30% vapor for the spark to start a cold engine.
That's what the ECT(engine coolant temperature) sensor is for.
ECT sensor is only connected to computer, in 2003 and earlier.
There is also a SENDER near the ECT sensor, it is only used by the dash board temp gauge.
Around 2005 they did combine these, well they actually got rid of the SENDER, and the Computer sent engine temp to the dashboard temp gauge.

So the ECT sensor tells the Computer how much Choke(extra fuel) to apply to the cold engine, gasoline's vapor % can be calculated based on temp, as said you can look that up, Ford did, lol, so the computer knows how much extra gasoline to squirt into the engine to get the correct amount of vapor in the cylinder for spark to start the engine at any temperature.

So, yes check connector on ECT Sensor, it is an important sensor for cold starts, must have accurate temperature.
ECT sensor is monitored so would usually cause CEL(check engine light) if there was a problem.
But Computer can only monitor that it is changing as engine heats up, it can't tell if it is accurate for 20degF or 30degF before startup

And yes do the key on/key off a few times before starting, as a loss of fuel pressure when vehicle sits is not uncommon and more likely than the ECT sensor issue.
But since you just worked on that area of the engine could be ECT

It's been almost a year, (EDIT it's been almost 2 years....), since this happened, but I was so upset, that I totally forgot about this post yall and I apologize. But, yes, it was the 2 sensors, that were bad. I honestly would not have thought that changing thermostat housings would've needed those 2 sensors replaced, but it's definitely something I'll remember in the future, if it ever happens again!

kimcrwbr1 · Jan 19, 2019

The 1 wire sender is for the temp guage and the two wire is for the ecm. The chances both were bad is slim to none. There are ohm tests you can take to determine a faulty sender/senser. Only the two wire senser will effect engine performance!

adsm08 · Jan 19, 2019

Mirage775 said:
It's been almost a year since this happened

In another 6 weeks it will be two years.

RonD · Jan 19, 2019

Mirage775 said:
It's been almost a year since this happened, but I was so upset, that I totally forgot about this post yall and I apologize. But, yes, it was the 2 sensors, that were bad. I honestly would not have thought that changing thermostat housings would've needed those 2 sensors replaced, but it's definitely something I'll remember in the future, if it ever happens again!

Good work

Mirage775 · Jan 19, 2019

kimcrwbr1 said:
The 1 wire sender is for the temp guage and the two wire is for the ecm. The chances both were bad is slim to none. There are ohm tests you can take to determine a faulty sender/senser. Only the two wire senser will effect engine performance!

They were both cheap and were in stock at Autozone, so I just replaced both

adsm08 said:
In another 6 weeks it will be two years.

Damn, you're absolutely right... I keep thinking I did this last year, for some reason, LOL!

RonD said:
Good work

Thanks!

2003 4.0, Replaced Thermostat Housing, now, it's hard to start only when cold?

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